Liquid enzyme solutions are now being used as breakers in hydraulic fracturing and horizontal drilling applications in the oil and gas industry. One of the requirements of an enzyme used as a breaker in a subterranean formation is that it retains its activity at high pressure, such as at temperatures reaching up to 150 degrees Celsius and at a pH ranging from 4 to 12.
Fracturing fluids are mixtures that contain various components, each with a purpose in the fracturing method. Fracturing fluids include drilling fluids, diverting fluids and gravel-packing fluids. Fracturing fluid components include proppants, such as sand, silica, glass beads, metal particulate matter, sintered bauxite and other ceramic proppants, ultra-light weight (ULW) and other matter, to ‘prop’ open subterranean fractures from which oil and gas flow into a wellbore and are recovered; viscosifying agents or gelling agents such as guar gum, xanthan gum and others, are added to increase the specific gravity of the fracturing fluid to carry proppants to the subterranean fractures; and breakers which reduce the viscosity of the viscosifying agents so that the oil and gas can flow into the wellbore. The breakers are important in maximizing the recovery of oil and gas from the well. The viscosifying agents are typically selected from galactomannan gums, guars, derivatized guars, cellulose, cellulose derivatives, starch, starch derivatives, xanthan, xanthan derivatives and mixtures thereof. While these viscosifying agents are effective at carrying sand or other proppants into the subterranean formations, they often form a rubbery solid with elastic properties called “filter cake” that reduces the flow of gas and/or oil from the subterranean formation. It is therefore useful to prevent formation of filter cake, or to dissolve the filter cake once it is formed. Breakers are a class of chemical that reduce the viscosity of fracturing fluid and prevent filter cake build-up.
Breakers are typically selected from three different classes of chemicals: oxidizers, enzymes and acids. Enzymes are useful as breakers because they are naturally occurring proteins that are specific to the viscosifying polymers listed above. Enzymes such as alpha-amylases, glucoamylases, xanthanases, xylanases, cellulases, hemicellulases, cellobiohydrolases, beta-glucanases, and others, hydrolyze the bonds that characterize viscosifying polymers. Enzymes are generally preferable from an environmental perspective, to oxidizers such as sodium persulfate, ammonium persulfate, chlorates, bromates, periodates and acids such as citric acid and fumaric acid among others.
Typically, viscosifying agents and proppants are pumped into a wellbore under sufficient pressure to cause the subterranean formation to fissure. The viscosifying agent ensures the proppant is carried into the fissure. Subsequently, the breaker is pumped into the wellbore. Breaker and viscosifying agents come into contact and the breaker reduces the viscosity of the fracturing fluid. It is difficult to ensure the breaker reaches the viscosifying agent and filter cake located in the fissures. In addition, since breakers are typically water soluble and there is significant fluid loss, in the subterranean formation, much of the breaker is lost with the fluid. The increased viscosity of the fracturing fluid also reduces the diffusion of the breaker in the formation. In order to overcome the loss and reduced diffusion, increased volumes of breaker are required which increases cost.
U.S. Pat. No. 8,343,747, and published U.S. Patent Application No. 20100011456 assigned to Verenium Corporation, disclose a series of genetic sequences that code for enzymes that can be used in hydraulic fracturing to reduce the viscosity of drilling mud and other viscosified treatment fluids used in subterranean formations. These documents do not teach or suggest that such enzymes can be immobilized on the proppant such as sand or silica as a delivery mechanism to co-locate the enzyme breaker with the viscosifying agent that the enzyme is designed to hydrolyze.
Cochet et al., U.S. Pat. No. 8,393,395, describes encapsulation of breakers, including enzymes in a water-insoluble matrix and pumping these into a subterranean formation for the purpose of delayed release of breaker to reduce viscosity of a viscosified fracturing fluid. While Cochet's method provides benefits in terms of delaying the viscosity reduction until proppant has entered the fracture, Cochet requires the addition of both the water-insoluble matrix and the proppant.
Abad et al., U.S. Pat. No. 7,677,311, describes a composition and method for breaking viscous fluids by providing a solid particle that can be located in a subterranean fracture then subsequently decomposing into a breaker for the viscosifying fluid. Abad's method also ensures that breaker is localized in the fracture where needed, at least initially. Sullivan et al., U.S. Pat. No. 7,287,590, describes a delayed breaker. However, a limitation of both the Abad and Sullivan methods is that once solubilized, the breaker could be carried back up to the well surface, thereby nullifying its ability to reduce viscosity in the formation. The solubilized breaker may also be lost within the porous formation itself.
U.S. Patent Application No. 20130112413 (Muthusamy et al.), discloses a method of controlled release of enzyme breakers for oil field applications. Muthusamy discloses a viscosified treatment fluid consisting of a gelling agent, a crosslinking agent, a proppant, an aqueous-base fluid and a poly(meth)acrylate encapsulant that encapsulates at least one of an enzyme, an oxidizer, a chelator and an acid. Muthusamy's encapsulation method suffers from the same problems as the Abad and Sullivan methods.
Powell, U.S. Pat. No. 7,195,071, discloses that a succinoglycan hydrolysis enzyme can be impregnated on a carrier for delayed release. This method is suitable when delayed release of enzyme is desired however the enzymes eventually become soluble and may be lost to the formation with other fluids, or inactivated over time as is often the case with soluble enzymes. In addition, any improved stability of the enzyme conferred by impregnating the enzyme on a carrier is likely reversed when the enzyme is released.
Both, U.S. Pat. No. 5,437,331 (Gupta et al.) and U.S. Pat. No. 7,000,701 (Todd) disclose a method of fracturing a subterranean formation by, inter alia, encapsulating an enzyme breaker. In the case of U.S. Pat. No. 5,437,331, the enzyme breaker uses open cellular encapsulation to protect and delay the action of the enzyme. Todd uses a partially hydrolyzed acrylic material as polymer for encapsulation.
Gupta, Todd, Powell, Muthusamy, Cochet and Abad do not teach or even suggest a method for retaining the breaker on the solid particle so that the breaker is not lost in the formation or when the petroleum fluids flow back to the surface of the earth. Moreover, these patents do not teach that the proppant can be used as an immobilization media and that in doing so, the fracturing method can be improved and simplified and, in certain cases, the enzyme breaker can be recovered and reused.
U.S. Pat. No. 7,021,379 (Nguyen) describes a method of forming subterranean fractures penetrated by a well bore and consolidating proppant particles therein where the particles are coated with a hardenable organic resin, a silane coupling agent and a gel breaker. Nguyen discloses the use of enzymes as gel breakers, among other chemicals such as oxidizers and acids. Nguyen's proppant requires three separate chemicals as part of the coating composition; a hardenable resin, a silane coupling agent and a gel breaker. Nguyen's method employs breakers that facilitate removal of gelled carrier fluid from the surface of hardenable resin-coated proppant particles allowing for resin coated particle-to-particle contact.
U.S. Pat. No. 6,186,235 (Tjon-Joe-Pin) discloses a method of forming a breaker-crosslinker-polymer complex where the breaker is in an inactive form prior to a change in subterranean conditions at which point the breaker is activated. Once the breaker begins to breakdown the gelled polymer complex, the soluble breaker can be lost through the formation. Tjon-Joe-Pin does not disclose the use of proppant as a solid phase upon which the breaker can be immobilized.
U.S. Pat. No. 5,998,183 (LeFevre et al.) and U.S. Pat. No. 7,312,056 (Saville and Khavkine) disclose methods of immobilizing enzymes on a support matrix for use in commercial chemical production methods, such as pharmaceuticals and syrups. Both of these immobilization methods do not disclose forming a proppant having an immobilized enzyme. The methods in these patents can be modified using the present invention to provide the formation of an improved proppant-immobilized enzyme described herein. The complete disclosures of U.S. Pat. Nos. 5,998,183 and 7,312,056 are incorporated herein by reference.